A holder element of this type, which is formed in a rotationally symmetrical manner, has been disclosed previously, see, for example, Swedish patent document SE 93.01406-6. They are principally intended to form a combined passage through the skin and controlled positional fixation of other equipment, for example equipment for transferring information to the inner ear in the case of persons having hearing difficulties.
In this context, the previously disclosed holder elements are based on the anchoring elements, or fixtures, which have been used with great success for supporting artificial teeth and teeth bridges, and also other prostheses such as artificial joints, in the case of joint reconstructions, etc. The above referenced patent publication describes just one example of such a holder element. In addition to the fact that the holder element described in the above Swedish patent document is formed in a rotationally symmetrical manner, it is provided with an external thread which extends from the insertion end up to the mounting end. The external thread increases the initial stability of the holder element during the implantation and, by means of providing the insertion end of the holder element with self-tapping slits, where the longitudinal edges of the slits are shaped as cutting edges, an increased grip is obtained against the walls in a hole which has previously been prepared in the bone tissue.
It is important for long-term anchorage stability that the material is tissue-friendly and preferably consists of titanium, with the titanium exhibiting a micropitted surface having a pit diameter of between 10 and 1000 nm, preferably 10 and 300 m. This creates the best conditions for a good, and consequently enduring, anchorage between the cell offshoots of the tissue and the micropits.
When the known holder element is being inserted by operation, the bone is exposed by dissection and a hole, whose depth corresponds to the intended depth of insertion and whose diameter principally corresponds to the internal diameter of the outer thread, is made in it. Since the holder is screwed into the prepared hole, leads and contact elements have to be passed into the holder and fixed there subsequently, since the leads would otherwise become twisted. The known holder element is therefore provided with a separate contact element whose position is fixed against an inner ring flange or shoulder. While it is true that it is stated that the contact element should sit well against the wall of the through bored-out hole, the additional gap which is formed between the contact element and the wall of the bored-out hole always constitutes a point of potential danger for bacterial ingrowth and for infections in association with passage through the skin.
It is difficult to fix the position of, and assemble, a contact element and other equipment in the holder element of the patient after the holder element has been mounted and there is a risk of the holder element being subjected to undesirable forces during the assembly so that the integration between the implant surface and the surrounding bone tissue is disturbed, resulting in there being a risk of the holder element becoming loose in the long term. There is, therefore, a need to produce a holder element which has as few parts as possible and which, when being mounted, is as preassembled as possible so that the risk of gaps and disturbances of the initial Stability is minimized.
When the holder element is used as a cochlear implant, it is placed in the bone behind the ear and includes equipment for transferring information to the inner ear. Contact leads pass through the through channel and are guided out through the insertion end of the holder and conducted to the inner ear. In this context, the contact leads have to be angled off at what is in principle 90.degree. after they have left the holder element in order to ensure that they do not come too close to the meninges and risk damaging them. Even if the mastoid bone behind the ear is somewhat thicker than the remainder of the cranial bone, there can be a lack of space for passing the contact leads through in this way. It is true that it is intimated that, in the case of the known holder element, it should be possible to guide the contact leads out from the through channel by way of openings which are arranged in the casing surface of the holder element. However, it is not shown how this could is be done in practice, since the holder is screwed into the bone and the hole in the casing surface must in that case be matched with a corresponding hole in the bone adjacent to the opening in the casing surface. Furthermore, it is desirable to leave the bone adjacent to the casing surface of the holder as undisturbed as possible, on the one hand in order to ensure that the barrier against bacterial leakage remains as thick as possible. On the other hand it is desirable in order to avoid unnecessarily diminishing the surface which adjoins the bone and which is to be integrated with the bone.